Much has been written about Machu Picchu since its rediscovery in 1911 by Hiram Bingham and his expedition crew. And although I was truly amazed at the ruins of Machu Picchu when I hiked around it a few months ago, I was mesmerized by the area geology as soon as I got off the train at Aguas Calientes – the town at the base of Machu Picchu. Consequently, it’s the geology of Machu Picchu that I’ll talk about in this blog rather than the ruins. But – for those who would still like to read more background information on Machu Picchu, the Library of Congress has a good online bibliography site for a starting point- Machu Picchu: A Brief Bibliography.
The geographic setting of Machu Picchu –
Machu Picchu lies in the south-central Cordillera of the Peruvian Andes, known as the Cordillera de Vilcambamba. Cusco, the nearest major city, lies about 50 miles southeast of Machu Picchu. Most sojourners like myself access Machu Picchu via the Sacred Valley either by train or by walking the Inca Trail, and stay in Aguas Calientes during their time exploring Machu Picchu.
The geologic setting of Machu Picchu –
As soon as I got off the train at Aguas Calientes, I could see that it was a granitic dominated geology. Large remnant exfoliation sheets, typical features of granitic landscapes, cling to the mountainsides in every direction that I looked. Canitu and others (2009, p.250) describe the geology of the the Machu Picchu site as:
“The bedrock of the Inca citadel of Machu Picchu is
mainly composed by granite and subordinately granodiorite.
This is mainly located in the lower part of
the slopes (magmatic layering at the top). Locally,
dikes of serpentine and peridotite are outcropping in
two main levels; the former is located along the Inca
trail, near Cerro Machu Picchu (vertically dipping),
the latter is located along the path toward ‘‘Templo de
la Luna’’ in Huayna Picchu relief.”
The granitoid pluton of Machu Picchu is part of the larger “Quillabamba granite”, which is a magmatic complex now exposed in the eastern Cordillera of central Peru. The Machu Picchu pluton, along with numerous other areal plutons of this magmatic complex, were intruded into an axial zone of a Permo-early Jurassic rift system. Isotopic age data that more tightly constrain this magmatic activity include a (U–Pb) age of 257 +3 My for the Quillabamba granite and a biotite Rb-Sr age of 246 + 10 My for the Machu Picchu pluton (from Lancelot and others, 1978: U/Pb radiochronology of two granitic plutons from the eastern Cordillera (Peru) — Extent of Permian magmatic activity and consequences. Int. Journal of Earth Sciences, 67(1), 236–243). The current exposure of the Machu Picchu pluton at such a high elevation is due to a tectonic inversion of the rift system’s axial zone. The inversion is a result of Andean convergent deformation that occurred largely during the Eocene (Sempere and others (2002) cited in: Mazzoli and others, 2009).
The site-specific geologic structural setting of Machu Picchu is that the citadel ruins lie within a northeast-trending graben. The graben is delineated by two normal faults with the upthrown side on the northwest including Huayna Picchu and the upthrown side on the southeast being the block that contains Machu Picchu Cerro. As an aside, there are great 1-3 hour hikes that can be done, both to Huayna Picchu and to Machu Picchu Cerro. I did the hike to
Huayna Picchu with a great group of people, so it was a fun hike made even better by spectacular views from the top of Huayna Picchu.
Building stone of Machu Picchu –
Lastly, because the ashlar method of stone block construction (a method where stone blocks are dry fit together so well that it is impossible to slide a piece of paper between the blocks) used in Inca architecture is so fascinating, I’ll include a few words about the stone used in this method at Machu Picchu.
The building stone of the Machu Picchu citadel ruins was quarried from the area granitoid rocks. Canuti and others (2009, p. 256) in their study of Machu Picchu slope instability note that:
“As historical consideration, the data collected
suggest the possibility that the site of Machu Picchu
could have been selected by Incas also because of
the availability of two large block deposits, useful
for constructions: one on the so called ‘‘cantera’’
and the second in the paleo-landslide recently
The “cantera” mentioned above is the quarry that was used during the original construction of Machu Picchu. It is located between the Sacred Plaza and the Temple of the Sun at Machu Picchu. It looks like just a chaotic pile of rocks, so is probably not a point of interest for most visitors. The paleo-landslide also mentioned above as a potential source for granitic building material is an area located on the northeast flank of the Machu Picchu citadel ruins. Canuti and others (2009) suggest that it is probably some tens of meters thick and luckily their deformation monitoring did not detect mass movement.
And so ends my 5-part blog series on my adventures in Peru. All I can say is – go there if you get a chance. It is an amazing place!